The use of genetically-engineered mice and rats (e.g. transgenic, knock-out, knock-in, conditional transgenic, etc.) as animal models has become widespread in almost all aspects of biology. In many of the projects carried out by scientists within the IDDRC, genetically-engineered mice or rats are being used as animal models to determine the function of genes, fundamental biological mechanisms of cell/tissue function, and/or to use as animal models of disease or behavioral abnormalities. These animals have either been generated here at UCLA or they have been imported from other institutions and used to establish local breeding colonies. With this large increase in the availability and use of transgenic animals, there is a concomitant need for core services to help individual scientists create, breed, import, genotype and evaluate mouse and rat lines. Many, if not most, of the scientists obtaining these mice have not had to deal with breeding and genotyping of such animals before. Nonetheless, they need to be able to take advantage of these animal models to extend their research effort into important new areas. For these reasons, we have set up Core services to educate investigators about and perform these activities. The nature of the services for investigators will vary in their degree of sophistication and Core involvement for each investigator will depend upon the needs of individual laboratories.